1. Field of the Invention
This invention relates generally to the art of agglomeration and more particularly to methods of forming agglomerates of iron-bearing flue dust collected as a by-product from metallurgical processes for recycling the otherwise waste by-product.
2. Discussion of the Prior Art
Dust collectors in metallurgical making operations, such as in the making of iron and steel, collect large quantities of particulate matter which can be generally categorized as flue dust. Most of this material is discarded as waste. However, mush of it contains valuable material which can be profitably recycled in metallurgical processes.
Much of it is collected in the form of dry granules or dust from cyclones or bag houses. This dry material is presently being recycled by pelletizing or briquetting as described for example in U.S. Pat. Nos. 3,316,081; 1,158,364; 1,741,544; and 1,038,370.
When the flue dusts are collected in the form of a wet sludge which is generated from wet scrubber operations, the problem of recycling becomes much more difficult. In order to recycle these materials using conventional or known agglomerating technology, the wet sludge must first be dried down, which is an expensive and relatively difficult operation. After the material is dried down, it is then agglomerated in the fashion previously described with regard to dry dust collected from cyclones or bag houses. In either event, agglomeration of these dry dusts can also produce hazardous dust conditions in manufacturing the agglomerate, and the ultimate size of the agglomerate is not readily controllable and is generally too small for many furnace charge applications which require the free flow of air and oxygen throughout the furnace charge.
As an example of one such iron-bearing flue dust, B.O.F. flue dust is found in great abundance in steel making operations, either in dry form in cyclones or bag houses as collected from precipitators or in wet form in the form of a B.O.F. sludge which is discharged from a wet scrubber on a B.O.F. operation. The discharge from the wet scrubber is pumped into a thickening tank where most of the particulate matter settles out. This thickened material is then pumped into a vacuum filter where more of the water is removed. The resulting filtercake material is generally designated or called B.O.F. sludge, and contains about 30% moisture and has an approximate composition range as follows:
______________________________________ Iron oxides 65- 90% MnO 1-3% Manganese oxides 1- 4% Na.sub.2 O .01-4% Phosphorus .05- .2% K.sub.2 O .01-2% SiO.sub.2 1- 4% Pb .01-.6% Al.sub.2 O.sub.3 .1- 1% Zn .05-.5% CaO 4- 10% S .05-1% ______________________________________
Other flue dusts which contain valuable constituents for recycling may also be found as a by-product in dry or wet form from iron-bearing sludges or dust generated in steel making operations from cyclones, bag houses or wet scrubbers on blast furnaces, electric furnaces, and cupola furnaces. Of particular interest, due to the valuable manganese content, are dry or wet manganese-bearing dusts or sludges such as those collected from ferromanganese blast furnace operations.
In agglomerating these fine mill waste dusts for recycling, it is necessary to form agglomerates with sufficient strength such that the material can be handled without undue breakage. In addition, if these agglomerates are to be recharged to a blast, cupola, or electric furnace, it is also important that the agglomerates have adequate size and good load-bearing capacity, both at low and elevated temperatures. If they do not, the agglomerates will readily decompose in the furnace thereby creating fines which in turn diminishes the void spaces in the furnace charge. These voids are very important in such furnaces for gas flow for proper furnace operation. The recycling and agglomeration techniques of the prior art do not satisfactorily provide an economic method of agglomeration, nor do they satisfactorily provide resultant agglomerates which will meet these requirements.